This invention relates to a method of and apparatus for outgassing raw materials used to grow crystals and, more particularly, to such a method and apparatus useful when the grown crystal must be very pure. For example, in the electronics industry crystals are used as substrates for silicon to provide a wafer on which integrated circuits are formed. These crystal substrates must be free of internal voids and contaminants which adversely effect the electronic characteristics of the circuits.
Sapphire has become a widely used crystal as such a substrate. It is formed from aluminum oxide (Al.sub.2 O.sub.3) granules or powder which are fed to a heated growth chamber having an inert atmosphere such as argon. Any water, air or other contaminants in the granules or powder are released in the growth chamber and become entrapped in the resultant crystal forming internal voids.
In order to minimize this problem, attempts have been made to outgas the granules or powder that is, to purify it by driving out water, air and other contaminants, before it is fed to the growth chamber. Such attempts have not provided the relatively high purity required by the electronics industry and/or have been time consuming so that they are not generally satisfactory.
One such technique is generally disclosed in U.S. Pat. No. 3,998,686 issued Dec. 21, 1976 to Meiling et al. This patent discloses the use of a double walled, water cooled bell jar sealed by elastomer or metal o-ring seals. The aluminum oxide is placed inside the bell jar and is then evacuated to about 5.times.10.sup.-5 torr and heated to about 800.degree. C. for a period of twenty-four hours to bake out the contaminants. Thereafter, the aluminum oxide is maintained in an inert atmosphere, first helium, then argon. In practice, this technique is time consuming and suffers from several problems. Moreover, it eliminates only about ninety-five percent of the contaminants.